The rubber constitution of the tread is a major factor determining the running properties of a tire, in particular of a pneumatic tire. The rubber mixtures found in drive belts, hoses and other belts, especially in the locations subjected to high mechanical load, are likewise substantially responsible for stability and durability of the rubber products. These rubber mixtures for pneumatic tires, drive belts, other belts and hoses are therefore subject to very stringent requirements.
By way of example, the running properties of tires have been raised to a higher overall level in the past by partial or complete replacement of carbon black filler by silica in rubber mixtures. However, the known conflicting objectives relating to tire properties that have inversely correlated behavior continue to exist, even in tread mixtures comprising silica. By way of example, an improvement in wet grip and in dry braking generally continues to be associated with impairment of rolling resistance, of winter properties and of abrasion behavior. Good grip and low abrasion are also an important quality criterion in industrial rubber products, such as drive belts and other belts.
Many different approaches have been taken in attempts to resolve these conflicting objectives. By way of example, a very wide variety of polymers, resins and fine-particle fillers, where these include modified materials, have been used for rubber mixtures, and attempts have been made to influence vulcanizate properties by modifying the mixture-production process.
Another important class of additives, alongside rubber and fillers, is that of the plasticizers. Large amounts of plasticizers are sometimes added to rubber mixtures in order to reduce the cost of the mixture, to improve the flow properties of the mixture (energy saving during processing, avoidance of energy peaks), improve filler dispersion, improve compounding behavior and adhesion behavior, and influence the physical properties of the mixture and of the vulcanizates produced therefrom.
Alongside the aromatic, naphthenic and paraffinic mineral-oil plasticizers usually used in rubber mixtures, there are various synthetic plasticizers that are used in rubber mixtures, for example, thioesters, phthalic esters, aromatic polyethers, phosphoric esters, sebacic esters or low-molecular-weight, polymeric polyesters.
However, the production of these plasticizers is expensive and requires a large amount of energy. It moreover uses raw materials such as petroleum which will become unacceptable from an environmental point of view, and especially in respect of the current situation on emissions of hazardous materials and raw-material shortages. Attempts are being made, as an alternative, to use vegetable oils as plasticizers in rubber mixtures. However, the rubber industry does not have access to unlimited amounts of these.
The following publications may be mentioned by way of example of a large number of others relating to the use of the abovementioned plasticizers: EP 0 708 137 A1; US 2002/0010275 A1; US 2002/0042462 A1; U.S. Pat. No. 4,737,535; DE 101 08 981 A1; and, U.S. Pat. No. 7,335,692.
European patent application 0 708 137 A1 describes vulcanizates which comprise no aromatic process oils, but comprise at least one plasticizer (for example, dioctyl phthalate), liquid polymer and/or asphaltenes and, as filler, silica and carbon black in a ratio of from 1:1 to 20:1, the aim being to avoid any occurrence of black markings caused by abrasion and by discoloration in the original rubber mixture.
United States patent application publication 2002/0010275 A1 discloses addition of from 1 to 20 phr of a low-molecular-weight polyester plasticizer selected from polyester sebacate, triethylene glycol caprate-caprylate, triethylene glycol diheptanoate, triethylene glycol dipelargonate and triethylene glycol di-2-ethyl hexoate to a tread rubber mixture, the intention being to soften the tread and to improve the traction behavior of high-performance tires.
United States patent application publication 2002/0042462 A1 discloses rubber mixtures for tires which comprise reinforcing fillers and an ester, the aim being to improve processability without disadvantages in other properties. The ester here is one selected from an ester of an aliphatic, polybasic carboxylic acid with a polyoxyalkylene derivative and an ester of an aromatic, polybasic carboxylic acid with a polyoxyalkylene derivative.
U.S. Pat. No. 4,737,535 describes tires intended to have improved adhesion properties, steering stability properties and low-temperature properties. The tires have treads made of a rubber mixture, which comprises from 15 to 50 phr of SSBR, at least one plasticizer selected from a sebacic ester, an adipic ester and a fatty acid ester, and carbon black.
German patent publication 101 08 981 A1 and U.S. Pat. No. 7,335,692 disclose the use of proteins made from oil seed, or sunflower oil, as plasticizers in rubber mixtures, particularly for pneumatic tires. The vegetable oils described in those documents can be used as sole plasticizer but are mostly used in combination with another plasticizer obtained from petroleum.